The method of forming an air spring as set forth in U.S. Pat. No. 3,057,768 is exemplary of the state of the art. The flexible member blank is first cylindrical and is then conically expanded at one of its ends. For this purpose, a pressure collar is introduced into the flexible member blank and a rigid form having conical or bottle-shaped spaces is applied and, after the two ends have been closed, the pressure collar is charged with hot vapor. The blank assumes the outer shape which is wanted because of the pressure and heat and is vulcanized, that is, the two ends have different diameters.
Whereas clamping the end of the flexible member blank which is not to be widened presents no difficulties, the clamping of the end which is to be expanded is not easily possible and requires special measures.
Also, the precision with respect to form of the end of the blank to be expanded could be better. In practice, it has been shown that the clamp ends are not usable for further use and must therefore be separated, that is, the blank must be cut at both ends to an extent which cannot be considered negligible.